There are two general techniques employed for obtaining a person's breath for gas analysis. In a first technique a person can cooperatively breathe into an instrument, which receives the gas for analysis. In a second technique an instrument can obtain a gas sample from the person's airway independent of the person's cooperation. In either technique, achieving a precise collection and precise analysis of a gas from a specific portion of the breathing cycle can be challenging, given the often random and erratic nature of a breathing pattern. For example, reliably measuring breath CO at exactly the end-tidal portion of exhalation, with high levels of accuracy and precision (for example <0.5 ppm accuracy), has proven difficult. Typically, a measurement of breath CO2 is used to determine the end-tidal portion of the breath, and gas from that portion of the breath can be sampled and analyzed accordingly. Using an end-tidal CO2 signal is a convenient approach in that the technology is well known, and provides an instantaneous measurement of the breath waveform. However, in order to obtain accuracy and precision in the remainder of the overall system, the instrumentation considers only some of the possible external factors that may be useful.
Typically, a constant flow rate is employed for withdrawing the gas from the person, for a fixed collection time, and placing the drawn sample in a fixed volume sample compartment. When using this approach, there may be breath pattern related inaccuracies. For example, some of the sample compartment may have non-end-tidal gas in it, or only a fraction of the end-tidal portion of the breath might get sampled and stored in the sample compartment for analysis. While the repeatability at a certain breath rate is very good, the accuracy may vary as the breath rate varies, because of the pneumatics and timing of the system.
Therefore, it may be beneficial to improve on the accuracy of known systems in a way that is equally accurate across a range of breath patterns and breath rates. To this end, various approaches and algorithms have been conceived and described herein.